The determination of receptor affinity constants of opiate agonist and antagonist agents provides important quantitative information for the differentiation of postulated subclasses of opiate receptors which mediate different narcotic actions. The overall objective of this project is the employment of improved methods for determining in vivo affinity, expressed as pA2, for naloxone-morphine. To this end, we shall use two distinct narcotic effects in the rat, analgesia and altered seizure threshold, each effect believed to be mediated by different receptors. Additional ojectives are: (1) to test the constancy of each pA2 through perturbations produced by the concomitant use of a second agonist with central action, and (2) to determine the pA2 for each effect in the morphine-tolerant animal. Current methods for determining pA2 for each lack accuracy and precision because they use administered doses instead of (the theoretically correct) brain concentration and, therefore, one has to make several dubious pharmacokinetic assumptions related to both the experimental procedure and the data analysis. Our approach will have the experimental precision of in vivo work since it uses sensitive techniques to measure brain concentrations. Two routes of administration will be used: subcutaneous and intracerebroventricular. Also, we shall use both the conventional (steady-state) method of calculating pA2 and a new time-dependent method which uses concentration-time-effect data. A further improvement in the accuracy and precision of the calculated pA2 values will be afforded by using methods of data analysis that comply strictly to competitive theory through the use of constrained data plots, a procedure heretofore neglected in the determination of pA2 in vivo. The information to be gained from this study will complement that derived from both narcotic profile studies using different agents and radioligand binding studies.